Processes for the catalytic dehydrogenation of an unsaturated hydrocarbon (hereinafter “first unsaturated hydrocarbon”) to form an unsaturated hydrocarbon which has one olefinically unsaturated bond more than the first unsaturated hydrocarbon and otherwise an unchanged carbon skeleton, (hereinafter “second unsaturated hydrocarbon”) are well known in the art. An example of such a dehydrogenation is the dehydrogenation of an alkylaromatic compound to yield, as the desired, main product, the corresponding alkenylaromatic compound. Another example is the dehydrogenation of a mono-olefin to yield as the desired, main product a corresponding conjugated di-olefin. The dehydrogenation catalysts customarily used in such processes are iron oxide based catalysts.
In such processes the dehydrogenation does not occur without side reactions, which decreases the yield of the desired second unsaturated hydrocarbon and therefore changes the economy of the process unfavourably.
For example, in the dehydrogenation of an alkylaromatic compound, one such side reaction is the formation of coke on the catalyst, of which in additional effect is that it decreases the catalyst lifetime. Other side reactions involve the formation of an alkynyl-aromatic compound, a methylaromatic compound, and a de-alkylated aromatic compound. For example, in the dehydrogenation of ethylbenzene, the desired, main product is styrene and undesired byproducts are coke, phenylacetylene, toluene and benzene.
In view of the applicability and use of the alkenylaromatic compound, the alkynylaromatic compound is frequently at least partly removed from the product of the dehydrogenation. This removal requires a separate process step, typically involving hydrogenation to the alkenylaromatic compound, using a selective hydrogenation catalyst.
Analogous side reactions may occur in the dehydrogenation of a mono-olefin.
In the past, much work has been carried out with the object of improving the activity and the selectivity of dehydrogenation catalysts, and the efforts are still going on. However, so far it has proved to be very difficult to improve the activity of a dehydrbgenation catalyst without compromising the selectivity and to improve the selectivity without compromising the activity. Thus, the work has led to the development of so-called high activity dehydrogenation catalyst and so-called high selectivity dehydrogenation catalysts.
Compared with high selectivity dehydrogenation catalysts, high activity dehydrogenation catalysts may be operated advantageously at a relatively low temperature. Alternatively, they may be operated at relatively high space velocity, saving reactor volume and catalyst inventory for a certain reactor throughput. However, these advantages are at the cost of selectivity. Compared with high activity dehydrogenation catalysts, high selectivity dehydrogenation catalysts offer a relatively high selectivity, but they are operated at a relatively high temperature or at a relatively low space velocity.
U.S. Pat. No. 3,223,743 discloses a dehydrogenation process wherein the dehydrogenation feed is contacted first with a high selectivity dehydrogenation catalyst and subsequently with a high activity dehydrogenation catalyst, with the object of improving the conversion and yield of the desired dehydrogenation product. The teaching of U.S. Pat. No. 3,223,743 has found worldwide application in commercial dehydrogenation units. U.S. Pat. No. 3,223,743 is incorporated herein by reference.
As used in this patent document, the term “conversion” means in a quantitative sense the fraction, in % mole, of the first unsaturated hydrocarbon which is converted. The term “selectivity” means herein the fraction, in % mole, of converted first unsaturated hydrocarbon which yields the second unsaturated hydrocarbon.
The performance of dehydrogenation catalysts may be measured by catalyst testing methods. As used herein, the term “temperature parameter” means the test temperature, in ° C., at which the catalyst provides under isothermal testing conditions a conversion of 70% mole, and the term “selectivity parameter” means the selectivity then achieved. It is noted that, as the temperature parameter is a temperature at which a certain conversion (viz. 70% mole) is achieved, the temperature parameter is lower as the activity of a catalyst is higher.
High activity dehydrogenation catalysts have a relatively low temperature parameter and relatively low selectivity parameter. High selectivity dehydrogenation catalysts have a relatively high temperature parameter and relatively high selectivity parameter.